Sodium cyanide is typically produced by reacting hydrogen cyanide with aqueous caustic soda, followed by evaporative crystallization to produce a slurry of sodium cyanide crystals. The sodium cyanide crystals are separated from the slurry, dried and usually formed by dry compression methods into briquets. The resulting briquets are about 99 weight percent anhydrous sodium cyanide.
The briquets are shipped to consumers generally in containers designed to exclude exposure to atmospheric air since the anhydrous sodium cyanide is very hygroscopic and can absorb substantial quantities of water when exposed to atmospheric air. If exposed to atmospheric air, serious difficulties in shipping and storage can result due to caking. Also, there is the added cost of excluding atmospheric air.
The majority of the consumers of this product prefer to feed the sodium cyanide into their processes in the form of an aqueous solution. They dissolve the sodium cyanide, often after breaking the briquets into smaller particles, to produce a solution containing about 20 to 25 weight percent sodium cyanide. To avoid hydrolysis with the resulting evolution of hazardous hydrogen cyanide vapors, an alkali such as sodium hydroxide is added to the dilution water. Sufficient alkali should be added to raise the resulting solution pH to above 12.
To avoid difficulties and cost associated with storage of the anhydrous product and industrial hygiene hazards due to generation of respirable dust when handling anhydrous sodium cyanide or breaking it into smaller particles, some of the larger consumers have changed to direct solution storage. This has been accomplished by dissolution of the briquets in the shipping container, usually tank trucks or railway cars, and unloading the resulting solution into storage tanks.
These shipping processes have the disadvantages of added investment and operating cost associated with concentrating, separating, drying and compacting anhydrous sodium cyanide only to dissolve and dilute it prior to use. It would thus appear that since the consumer uses dilute solutions, direct shipment of solution particularly where the sodium cyanide production facilities are located in reasonable proximity of the consumer would be appropriate. However, the shipment of liquid presents an unacceptable environmental risk of spills in the event of an accident during transportation.
The present invention eliminates the need to separate, dry and compact the sodium cyanide prior to shipment, yet it produces a solid product which is desired for safe handling. The sodium cyanide concentration is high enough that the cost of shipping water does not become an overriding concern. Also, consumers do not have dust hazards associated with handling the anhydrous sodium cyanide and do not have to add caustic to avoid hydrolysis.
The process comprises conversion of a slurry of specified maximum water content to a substantially solid ice-like hydrate of sodium cyanide, NaCN.2H.sub.2 O, or a mixture of anhydrous and dihydrate sodium cyanide. On arrival at the consumer's plant, the solidified product is delivered to the consumer's solution storage by addition of water and heat.